This invention relates to machines for fabricating integrated circuit packages; and more particularly, it relates to machines for filling a plurality of via holes in flat green ceramic substrates with a conductive ink.
Integrated circuit packages are conventionally made of several laminated flat layers of ceramic. Typically, the bottom layer is a solid rectangle, and the top layers are rectangles with a hole punched through their center. An integrated circuit die is placed in the hole of the top layers and attached to the bottom layer. Bonding wires make electrical connections between signal pads on the die and a set of conductors that are on the face of an internal layer near the hole. Signals on those conductors are routed to the top ceramic layer by conductor filled via holes which penetrate through the top and internal ceramic layers. Then another set of conductors on the top ceramic layer routes the signals from the filled via holes to stiff conductive leads which are suitable for mounting the package to a printed circuit board.
In the process of making the above-described integrated circuit package, the desired via hole pattern is punched into a flat green ceramic wafer. Subsequently these via holes are filled with a conductive ink. Thereafter the desired conductor pattern is screened onto the surface of the green ceramic; the various green ceramic layers that make up the package are placed on top of one another and are squeezed together under high pressure; and the composite laminated structure is hardened by subjecting it to a high temperature environment.
This invention describes a new and superior machine for carrying out the above-described step of filling the via holes in the green ceramic wafer with conductive ink. Prior to this invention, the inventor utilized a "squeegee" to manually fill the via holes. With this method, which is herein referred to as method #1, the green ceramic wafer is placed on a table; a via fill mask that has via holes which correspond to the via holes of the wafer is placed on the wafer; a blob of conductive ink is placed on the via fill mask; and the squeegee is used to manually press the ink through the via fill mask and into the via holes of the ceramic wafer by moving the squeegee back and forth with a rolling-pin type motion. An actual squeegee is made of urethane, and is about three-eighths of an inch square and six inches long. Via holes which are filled by this method, however, are deficient since they are non-hermetic and have enlarged elliptical-shaped tops as described in the Detailed Description in conjunction with FIG. 5.
Another via hole-filling method which the inventor used and which is herein called method #2 involves providing a hollow cylinder which is partially filled with the ink. One end of the cylinder is covered with a mask that has via holes which correspond to the via holes of the green ceramic wafer; and the other end of the cylinder is fitted with a piston. In operation, the cylinder is positioned such that the mask lies on the wafer and the corresponding via holes are aligned; and thereafter, the piston is pushed to squeeze a portion of the conductive ink out of the cylinder and into the via holes of the wafer. However, this method is deficient since it too produces non-hermetic via holes with enlarged tops as described in the Detailed Description in conjunction with FIG. 6.
Accordingly, a primary object of the invention is to provide a machine for filling via holes of a green ceramic wafer with conductive ink in a superior fashion.
Another object of the invention is to provide a machine for filling via holes in a green ceramic wafer such that the filled holes are hermetic.